Many displays, including electrophoretic displays, magnetophoretic displays, toner displays and potentially other electrowetting-based displays utilize microcells filled with a display medium. The display medium may include ink, toner or other materials that change light reflectance or transmittance properties based on applied electric or magnetic fields. In some displays each microcell represents an image pixel. In high resolution systems, each pixel, and thus each microcell is extremely small. Sealing these extremely small microcells has been a challenge.
In the past, various sealing methods have been used. In one method, a liquid polymer overcoating is spread onto a liquid display material filled cells. The overcoating hardens to form a cell cap sealing in the display medium. In an alternate embodiment, a liquid display medium is mixed with a sealing media as described in U.S. patent application Ser. No. 10/237,522 entitled ‘Electrophoretic Display and Novel Process for its Manufacture’ which is hereby incorporated by reference. The mixture is deposited in a cell. Over time, the two media undergo a phase separation with the less dense sealing media resting over the display media. After phase separation and curing, the sealing material seals the display material in a cell.
One problem with the described sealing methods is that liquid based sealing methods are sensitive to the display material and sealing fluid surface tensions as well as the surface energy of adjacent cell walls. A second problem is that the described sealing method is unsuitable for powders, empty cells, and other non-liquid based display materials. A third problem is that sealing polymers may be hygroscopic, the presence of moisture causing display degradation.
An alternative sealing method is to laminate a cover sheet over the cells. However, lamination usually involves a liquid or viscous adhesive. Without careful controls, the liquid adhesive may displace ink in the cells or interact with the display fluid.
Conventional pressure sensitive films based on tackifiers have also been used to seal the display. The term “pressure sensitive” indicates that the adhesive bond is formed on contact, without wetting, heating, or adding a curing agent. The term ‘tack’ defines the degree to which a pressure sensitive adhesive feels sticky or highly adhesive. However, the sticky adhesive may interact with display fluids. In particular, the small size of the cells makes it difficult to confine the sticky adhesive only to regions of contact with the cell walls; thus the sticky adhesive also covers the cell opening and contacts the display fluid creating problems. For example, in an electrophoretic display, ink particles in the display may adhere to the tacky adhesive compromising display quality. The display fluid may also diffuse into the adhesive polymer or swell the adhesive polymer. These effects can substantially degrade display material functionality and thereby reduce display contrast.
Additional problems include ‘creep’ which is a slow movement of the adhesive or backing under stress. Creep is highly undesirable in applications such as flexible displays.
Thus an improved method of sealing a display cavity is needed